7,120 research outputs found
Nitrous oxide emissions from 2008 to 2012 for agricultural lands in the conterminous United States
The soil N2O emissions data for the conterminous United States were generated by the DayCent ecosystem model using the crop and land-use histories for survey locations in the USDA-NRCS National Resources Inventory (NRI). The model also requires weather and soils data. Daily maximum/minimum temperature and precipitation data are based on gridded weather data from the PRISM Climate Data product. Soils data are obtained from Soil Survey Geographic Database (SSURGO). See Del Grosso et al. (2022) and US-EPA (2020) for more details about the simulations. Atmospheric inversions were conducted using the CarbonTracker Langrage framework (Nevison et al. 2018). These results provide total N2O fluxes for the domain using atmospheric observations and an inverse modeling, and are compared to the DayCent emissions to confirm seasonal patterns, particularly the role of freeze-thaw events in driving pulses of N2O emissions from agricultural lands.Nitrous oxide (N2O) is an important greenhouse gas (GHG) that also contributes to depletion of ozone in the stratosphere. Agricultural soils account for about 60% of anthropogenic N2O emissions. Most national GHG reporting to the UN Framework Convention on Climate Change assumes nitrogen (N) additions drive emissions during the growing season, but soil freezing and thawing during spring is also an important driver in cold climates. We show that both atmospheric inversions and newly implemented bottom-up modeling approaches exhibit large N2O pulses in the northcentral region of the United States during early spring and this increases annual N2O emissions from croplands and grasslands reported in the national GHG inventory by 11%. Considering this, emission accounting in cold climate regions is very likely under-estimated in most national reporting frameworks. Current commitments related to the Paris Agreement and COP 26 emphasize reductions of carbon compounds. Assuming these targets are met, the importance of accurately accounting and mitigating N2O increases once CO2 and CH4 are phased out. Hence, the N2O emission under-estimate introduces additional risks into meeting long term climate goals.US Forest Service 18-CR-11242305-109, US Department of Agriculture (USDA) UV-B Monitoring and Research Program, Colorado State University, under USDA National Institute of Food and Agriculture Grant 2016-34263-25763, and the USDA GHG and DayCent modeling NACA agreements (58-3012-9-012 and 58-3012-1-015
The gap exponent of XXZ model in a transverse field
We have calculated numerically the gap exponent of the anisotropic Heisenberg
model in the presence of the transverse magnetic field. We have implemented the
modified Lanczos method to obtain the excited states of our model with the same
accuracy of the ground state. The coefficient of the leading term in the
perturbation expansion diverges in the thermodynamic limit (N --> infinity). We
have obtained the relation between this divergence and the scaling behaviour of
the energy gap. We have found that the opening of gap in the presence of
transverse field scales with a critical exponent which depends on the
anisotropy parameter (Delta). Our numerical results are in well agreement with
the field theoretical approach in the whole range of the anisotropy parameter,
-1 < Delta < 1.Comment: 6 pages and 4 figure
Nonequilibrium spin transport on Au(111) surfaces
The well-known experimentally observed \textit{sp}-derived Au(111) Shockley
surface states with Rashba spin splitting are perfectly fit by an effective
tight-binding model, considering a two-dimensional hexagonal lattice with
-orbital and nearest neighbor hopping only. The extracted realistic band
parameters are then imported to perform the Landauer-Keldysh formalism to
calculate nonequilibrium spin transport in a two-terminal setup sandwiching a
Au(111) surface channel. Obtained results show strong spin density on the
Au(111) surface and demonstrate (i) intrinsic spin-Hall effect, (ii)
current-induced spin polarization, and (iii) Rashba spin precession, all of
which have been experimentally observed in semiconductor heterostructures, but
not in metallic surface states. We therefore urge experiments in the latter for
these spin phenomena.Comment: 5 pages, 3 figures, to be published in Phys. Rev.
Recommendations for the Determination of Nutrients in Seawater to High Levels of Precision and Inter-Comparability using Continuous Flow Analysers
The Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) brings together scientists with interests in physical oceanography, the carbon cycle, marine biogeochemistry and ecosystems, and other users and collectors of ocean interior data to develop a sustained global network of hydrographic sections as part of the Global Ocean Climate Observing System. A series of manuals and guidelines are being produced by GO-SHIP which update those developed by the World Ocean Circulation Experiment (WOCE) in the early 1990s. Analysis of the data collected in WOCE suggests that improvements are needed in the collection of nutrient data if they are to be used for determining change within the ocean interior. Production of this manual is timely as it coincides with the development of reference materials for nutrients in seawater (RMNS). These RMNS solutions will be produced in sufficient quantities and be of sufficient quality that they will provide a basis for improving the consistency of nutrient measurements both within and between cruises.
This manual is a guide to suggested best practice in performing nutrient measurements at sea. It provides a detailed set of advice on laboratory practice for all the procedures surrounding the use of 1
gas-segmented continuous flow analysers (CFA) for the determination of dissolved nutrients (usually ammonium, nitrate, nitrite, phosphate and silicate) at sea. It does not proscribe the use of a particular instrument or related chemical method as these are well described in other publications.
The manual provides a brief introduction to the CFA method, the collection and storage of samples, considerations in the preparation of reagents and the calibrations of the system. It discusses how RMNS solutions can be used to “track” the performance of a system during a cruise and between cruises. It provides a format for the meta-data that need to be reported along side the sample data at the end of a cruise so that the quality of the reported data can be evaluated and set in context relative to other data sets.
Most importantly the central manual is accompanied by a set of nutrient standard operating procedures (NSOPs) that provide detailed information on key procedures that are necessary if best quality data are to be achieved consistently. These cover sample collection and storage, an example NSOP for the use of a CFA system at sea, high precision preparation of calibration solutions, assessment of the true calibration blank, checking the linearity of a calibration and the use of internal and externally prepared reference solutions for controlling the precision of data during a cruise and between cruises. An example meta-data report and advice on the assembly of the quality control and statistical data that should form part of the meta-data report are also given
Electronic structure of the substitutional vacancy in graphene: Density-functional and Green's function studies
We study the electronic structure of graphene with a single substitutional
vacancy using a combination of the density-functional, tight-binding, and
impurity Green's function approaches. Density functional studies are performed
with the all-electron spin-polarized linear augmented plane wave (LAPW) method.
The three dangling bonds adjacent to the vacancy introduce
localized states (V) in the mid-gap region, which split due to the
crystal field and a Jahn-Teller distortion, while the states
introduce a sharp resonance state (V) in the band structure. For a planar
structure, symmetry strictly forbids hybridization between the and the
states, so that these bands are clearly identifiable in the calculated
band structure. As for the magnetic moment of the vacancy, the Hund's-rule
coupling aligns the spins of the four localized V, V, and the V electrons resulting
in a S=1 state, with a magnetic moment of , which is reduced by about
due to the anti-ferromagnetic spin-polarization of the band
itinerant states in the vicinity of the vacancy. This results in the net
magnetic moment of . Using the Lippmann-Schwinger equation, we
reproduce the well-known decay of the localized V wave function
with distance and in addition find an interference term coming from the two
Dirac points, previously unnoticed in the literature. The long-range nature of
the V wave function is a unique feature of the graphene vacancy and we
suggest that this may be one of the reasons for the widely varying relaxed
structures and magnetic moments reported from the supercell band calculations
in the literature.Comment: 24 pages, 15 figures. Accepted for publication in New Journal of
Physic
Chaos-driven dynamics in spin-orbit coupled atomic gases
The dynamics, appearing after a quantum quench, of a trapped, spin-orbit
coupled, dilute atomic gas is studied. The characteristics of the evolution is
greatly influenced by the symmetries of the system, and we especially compare
evolution for an isotropic Rashba coupling and for an anisotropic spin-orbit
coupling. As we make the spin-orbit coupling anisotropic, we break the
rotational symmetry and the underlying classical model becomes chaotic; the
quantum dynamics is affected accordingly. Within experimentally relevant
time-scales and parameters, the system thermalizes in a quantum sense. The
corresponding equilibration time is found to agree with the Ehrenfest time,
i.e. we numerically verify a ~log(1/h) scaling. Upon thermalization, we find
the equilibrated distributions show examples of quantum scars distinguished by
accumulation of atomic density for certain energies. At shorter time-scales we
discuss non-adiabatic effects deriving from the spin-orbit coupled induced
Dirac point. In the vicinity of the Dirac point, spin fluctuations are large
and, even at short times, a semi-classical analysis fails.Comment: 11 pages, 10 figure
Construction and Assembly of the Wire Planes for the MicroBooNE Time Projection Chamber
In this paper we describe how the readout planes for the MicroBooNE Time
Projection Chamber were constructed, assembled and installed. We present the
individual wire preparation using semi-automatic winding machines and the
assembly of wire carrier boards. The details of the wire installation on the
detector frame and the tensioning of the wires are given. A strict quality
assurance plan ensured the integrity of the readout planes. The different tests
performed at all stages of construction and installation provided crucial
information to achieve the successful realisation of the MicroBooNE wire
planes.Comment: 24 pages, 22 figures, accepted for publication as Technical Report in
JINS
Dissecting X-ray-emitting Gas around the Center of our Galaxy
Most supermassive black holes (SMBHs) are accreting at very low levels and
are difficult to distinguish from the galaxy centers where they reside. Our own
Galaxy's SMBH provides a uniquely instructive exception, and we present a
close-up view of its quiescent X-ray emission based on 3 mega-second of Chandra
observations. Although the X-ray emission is elongated and aligns well with a
surrounding disk of massive stars, we can rule out a concentration of low-mass
coronally active stars as the origin of the emission based on the lack of
predicted Fe Kalpha emission. The extremely weak H-like Fe Kalpha line further
suggests the presence of an outflow from the accretion flow onto the SMBH.
These results provide important constraints for models of the prevalent
radiatively inefficient accretion state.Comment: 18 pages, 5 PDF figures, pdflatex format; Final version, published in
Scienc
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